When Delhi-based Baldev Kumar Puri started off as a cub detective in 1986, infidelity investigations were rare. But now he, and scores of snoops in the country, counts matrimonial surveillance as their bread and butter.

Puri, whose father was also a detective, says he started his career with corporate investigations. "Earlier cases were restricted to the elite class - only they had the money to spend on investigations. Now, everyone comes to us, when they see signs of straying. " Puri's agency, called AMX Detectives Pvt Ltd, gets up to 50 cases a month.

The frequency of cases of infidelity, in fact, makes an appearance in a recent book, The Masala Murder, by Madhumita Bhattacharya, whose protagonist steadily gets disillusioned with being a detective when she has to deal with a huge pile of such cases.

"A lot of private detectives in India and abroad have to deal with this. Especially now with internet dating, and the Shaadi. coms of the world, the feeling among women is that they don't know the person they are marrying. And since elders are not performing their traditional role of groom vetting, women are going to detectives for it, " says Bhattacharya.

Most of those collecting evidence against their partners' transgressions are women, with about 75% married and the rest in committed relationships. "Unfortunately, " says Sanjay Singh, owner of the Delhi-based Indian Detective Agency, "Ninety per cent of the suspicions turn out to be correct. Most people have pretty strong suspicions about their partners and are mostly right but they don't want to gather the evidence themselves. " So what are the tell-tale signs of a philandering spouse? Has he or she changed their hairstyle or dipped into the peroxide bottle for the first time? And check out that underwear : Did the wife recently decide she just had to have new lingerie? Or has the husband inexplicably traded his boxer shorts for bikini briefs?

Predictably, the role of technology is pretty significant in catching the errant partner. Get suspicious when the mobile phone is always busy or the pre-paid balance on phone keeps running out, say detectives. "More often than not, people get caught because of their text messages, BBMs, chats etc, " says Singh, whose agency earns 30-40 per cent of its revenue through such investigations. Checking out your spouse or partner's extra-curricular activities can cost anything from Rs 30, 000 to Rs 80, 000 depending on the complexity of the case or the number of days required. Many clients also use agencies to collect incriminating evidence they can use when they file for divorce.

The perception of a PI as a shifty-looking guy in a dark macintosh, hat and glasses is not true. For one thing, quite a few detectives are women. More than three quarters of Puri's staff is women, most of who are used as bait in honey traps. Apart from clients who want to catch their partners in flagrante, many also want to test the strength of their loyalty, says Puri.

Some detectives even stray into the territory of marriage counselling. Puri branched out into couples counselling in 2009, when he saw that many clients wanted to work on their marriages or relationships. This is because, in many cases, the person cheated on wants the marriage to survive.